The invention relates to a method of producing glass transmissive to ultraviolet radiation.
The class of glass compositions with which the present invention is lies within the range having the following limits, expressed in percent by weight:
______________________________________ SiO.sub.2 61 - 70 CaO 0 - 6 B.sub.2 O.sub.3 0.5 - 3.5 BaO 4 - 15 together Na.sub.2 O 8 - 10 MgO 0 - 5 6 - 15 K.sub.2 O 9 - 12 Al.sub.2 O.sub.3 1 - 5 ______________________________________
These glasses are characterized, in particular, by a high transmissivity to ultraviolet radiation of wavelength 253.7 nm, so that they can be used as envelopes for lamps which emit what is generally referred to as germicidal radiation.
To obtain a high transmissivity to ultraviolet radiation the glasses must be produced from highly pure raw materials. In addition, an organic reducing agent is added to the mixture in order to maintain the iron, which is a substantially inevitable impurity, in the divalent form, since trivalent iron has a strong absorption for ultraviolet radiation. During the irradiation of the glass by ultraviolet radiation the transmissivity of the glass to this radiation gradually decreases over time because of solarisation in which ferrous ions are gradually reconverted to ferric ions.
In the prior art production method of this class of glasses no refining agent was used, since the usual refining agents, arsenic trioxide and antimony trioxide, show strong absorption in the ultraviolet range. The quality of the glass produced in this manner is not very high, and it contains a large number of gas occlusions or "seeds".